(1) Field of the Invention
This invention relates to a sensor for measuring resistivity of ultrapure water used mainly for cleaning semiconductor wafers and, more specifically, to the composition of an alloy, with the aid of which the resistance to corrosion and oxidation of a metallic electrode member is improved.
(2) Description of the Related Art
In a process for manufacturing semiconductor wafers, even very fine dust or a little dirt is not allowed to get in, therefore a very severe cleaning of the wafers is required. A cleaning by using chemicals and ultrapure water has been employed as a method of the cleaning. That is, in a cleaning process of semiconductor wafers, surfaces of the semiconductor wafers are cleaned with various chemicals such as a mixed solution of dilute hydrochloric acid with hydrogen peroxide water and dilute hydrofluoric acid, and then the chemicals adhering to the surface of the wafer are cleaned off by using ultrapure water.
In the method of the cleaning described above, generally, the electrical resistivity of spent ultrapure water is measured so as to judge whether the chemicals are cleaned off from the surface of the wafer or not, and in this resistivity measurement a sensor for measuring resistivity having metallic electrode members has been used. As to the sensor, a pair of electrodes arranged facing with each other is immersed into a liquid to be measured, an alternating voltage is applied between both electrodes to detect a change in current flowing between the electrodes, thereby an electrical resistance of the liquid to be measured is measured.
In the cleaning process described above, the cleaning with ultrapure water may be carried out in the same tank, which has been used for the cleaning with the chemicals, and in such a case, when the chemicals are replaced by the ultrapure water, the electrode member described above may be exposed to the chemicals. Therefore, a metal such as titanium (Ti), stainless steel, Monel metal and platinum (Pt) is known as the electrode member.
Among electrode members consisting of the metal described above, when titanum is employed as the electrode, the titanium electrode is corroded by dilute hydrofluoric acid, the oxidation resistance deteriorates, the surface of the titanium electrode is oxidized during use and a film having significantly high electrical resistance is formed on the surface, in which the thickness of the film changes with time passing, the electrode property easily changes with a disturbance such as a temperature change, thereby causing a problem in the reliability as the electrode.
When stainless steel is employed as the electrode, stainless steel has poor resistance to acid, though a degree of the change with time passing is less compared to the titanium electrode, an increase in resistivity with time passing due to the oxidation of the surface of the electrode is inevitable, thereby the deterioration in the reliability during use for a long period of time is inevitable.
When Monel metal is employed as the electrode, Monel meral has poor resistance to acid, and when the chemical for cleaning containing dilute hydrochloric acid is used, the Monel metal electrode cannot be used for a long period of time.
To the contrary, since platinum is chemically very stable metal, the platinum electrode has no problem in terms of the performance and is very useful in a laboratory scale, however the platinum electrode is not feasible in a practical use because of its high cost.
A resistivity meter, in which Monel metal containing no chromium is used, is commercially available for use in cleaning with the chemical containing dilute hydrochloric acid. However, the resistivity meter is corroded by a cleaning fluid consisting of hydrochloric acid, hydrogen peroxide water and water, which is used for cleaning semiconductors, therefore the resistivity meter cannot be used for a process of cleaning semiconductors.
It is therefore an objective of the present invention to solve the above problem and to provide a metallic electrode member having excellent resistance to corrosion and oxidation, thereby providing a sensor for measuring resistivity, which enables stable measurement of the resistivity over a long period of time with simple maintenance.
In order to attain the above objective, the present invention is to provide a sensor for measuring resistivity having metallic electrode members for measuring resistivity of liquid, characterized in that the metallic electrode member consists of nickel-based alloy containing 19 to 24 wt % of chromium and 1 to 7 wt % of iron. According to the composition described above, the resistance to corrosion and oxidation of the electrode is significantly improved.